Common burst mode optical communication systems for conducting data communication through transmission of light signals by means of optical fibers are provided with a digital automatic power control loop (digital APC loop) that maintains a constant output light intensity of a light source. In said optical communication systems, it is commonplace to use a laser diode as the light source. The digital automatic power control loop is used for monitoring the output light source of the laser diode to adjust the bias current of the laser diode to a predetermined current value, such that the output light source of the laser diode can be maintained at a constant light intensity.
Referring to FIG. 1, there is shown a circuit structure diagram for conventional digital automatic power control loop. As shown in FIG. 1, the automatic power control loop 100 comprises a photo diode 11, an amplifier 12, a sample hold circuit 13, a comparator 14, a counter 15, a digital to analog (D/A) converter 16, a laser diode driver 17, and a laser diode 18.
Wherein, the photo diode 11 is used for sensing the light intensity of the laser diode 18 to generate a photoelectric signal (IS). The amplifier 12 is used for receiving and amplifying the photoelectric signal (IS) to generate an amplification signal (AS). The sample holding circuit 13 is used for receiving, sampling, and holding the amplification signal (AS) to generate a sample holding signal (SH).
The automatic power control loop 100 provides four different reference voltages R1, R2, R3, R4 (e.g., R1>R2>R3>R4), and sets a target value equal to the intermediate value between the reference voltage R2 and R3. The comparator 14 is used for receiving the sample holding signal (SH), and comparing the sample holding signal (SH) with each of the reference voltages R1, R2, R3, R4, respectively, to generate four count control signals (C1, C2, C3, C4) with digital forms; thereafter, the count control signals (C1, C2, C3, C4) will be transmitted to the counter 15.
When the sample holding signal (SH) is greater than the reference voltage R1, the comparator 14 generates the count control signals (C1C2C3C4=1100). When the sample holding signal (SH) is between the reference voltage R1 and R2, the comparator 14 generates the count control signals (C1C2C3C4=0100). When the sample holding signal (SH) is between the reference voltage R2 and R3, the comparator 14 generates the count control signals (C1C2C3C4=0000). When the sample holding signal (SH) is between the reference voltage R3 and R4, the comparator 14 generates the count control signals (C1C2C3C4=0010). When the sample holding signal (SH) is less than the reference voltage R4, the comparator 14 generates the count control signals (C1C2C3C4=0011).
The counter 15 comprises a default count value (N), and counts the count value (N) according to the count control signals (C1C2C3C4). The count value (N) is counted down by “2” when the count control signals (C1C2C3C4) are equal to 1100. The count value (N) is counted down by “1” when the count control signals (C1C2C3C4) are equal to 0100. The count value (N) is no counting operation to maintain the original value when the count control signals (C1C2C3C4) are equal to 0000. The count value (N) is counted up by “1” when the count control signals (C1C2C3C4) are equal to 0010. The count value (N) is counted up by “2” when the count control signals (C1C2C3C4) are equal to 0011.
Subsequently, the digital to analog converter 16 is used for receiving the count value (N) outputted from the counter 15, converting the count value (N) with a digital form into a count value (AN) with an analog form, and supplying the count value (AN) with the analog form to the laser diode driver 17.
The laser diode driver 17 is used for receiving the count value (AN), and thereby generates a bias current (Ibias) relative to the size of the count value (AN) to operate the laser diode 18 such that the light is emitted from the laser diode 18.
The automatic power control loop 100 is used for monitoring the light intensity of the laser diode 18 though the photo diode 11 to gradually adjust the feedback signal (e.g., the sample holding signal (SH)) to the target value, such that the bias current (Ibias) can also be adjusted to a predetermined current value. Thereafter, the light source of the laser diode 18 can be maintained at a constant light intensity so that the transmission of data in the optical communication system may be accurately.
Although, the automatic power control loop 100 of the prior art can adjust the bias current (Ibias) to a predetermined current value by the negative feedback. However, the sample holding signal (SH), which is used to indicate the light intensity of the laser diode 18, must be compared with the reference voltage R1, R2, R3, R4 simultaneously so as to observe whether the bias current (Ibias) is equal to the predetermined current or not. In such a way, the circuit complexity and power consumption of the comparator 14 will increase because of the complicated comparative procedure, which is unfavorable for the circuit development and circuit application of the automatic power control loop 100.